MTU and UPNP issues

[Vexira]

Hello all I seem to be having, issues with setting my MTU below, 1492, if I set it to a values like, 1446, for example, the router wont reconnect to the internet even when rebooted, it will only re connect if I set the mtu values back to 1492. it gets stuck on a disconnected status, my router is in full bridge mode with my dreytek vigor 130 modem. ive attempted this with a tp-link td 8817 in bridge mode with the same results. I'm using pppoe the main reason this worries me is if I do change isp's, back to one that hat a specific mtu values of 1460, as per their requirement.

Also, I'm unable to get both my pc's to have open nat status in call of duty black ops 3, open nat is where the game client is assigned the port number it requests and upnp has forwarded the port correctly.
The strange thing I've noticed is that one pc will be assigned, the port number, 2017 the default required port for open nat, but the other will receive the same port number internally but also a different external port number, resulting in moderate nat, rather than assigning a totally different port number, with in the udp port rage the game client will accept, I used to have this issue also when the game used port 3074 a few versions ago. instead of giving the second pc port 3075, it gave the game client on the other pc a different number instead. I assume this will happen if I buy the other call of duty games for my spare pc.
I have the minimum port range set to 1 and the maximum range set to default on both internal and external port range under pup in the wan tab.
When I attempted to use port triggering the port forward log was flooded with port numbers that the router attempted to use, including the values I set for triggering.
Ive also had the same issues on ASUS-WRT

I'm using Merlin 380.65 Beta -1 on my RT-AC88U
Just want to say thank you in advance for any help.

 

[Vexira]

If there is a way to fix the mtu issue I really would like to try it, since I'm trying to troubleshoot my Internet connection, and to me this issue is odd since, I used to have a netgear nighthawk x6 (r8000) and I could adjust the mtu with out an issue, but not on my 88u, I also tried to adjust the mount so the values are the same, but even after a power cycle the router still refuses to reconnect to the Internet, I also tried leaving the mru at 1492 and lowering the mtu but that didn't work either. The issue has been there ever since I got my 88u.
I can upload the sytem log if needed.